1. Field of the Invention
This invention relates to processes for the determination of thermal conductivity (or its converse, thermal resistance) of low density inorganic fiber mats by light transmission.
Inorganic fiber mats, particularly glass fiber mats in various thicknesses, are widely used both in construction and industry for thermal insulation purposes. The structure of such mats consists of fibers (often coated or treated with various materials) felted together in a bulky, low density open mesh network. Within this network are a myriad of minute dead air spaces bound by radiantly absorbing fiber surfaces.
It is these dead air spaces and surfaces that provide the major portion of the insulating property of the fiber mat. Because of this, thermal conductivity through the mat is a direct function of various physical properties of the mat, including the bulk (or "apparent") density, the predominant fiber orientation (if any), and the nominal fiber diameter.
"Apparent thermal conductivity," which is defined as the thermal conductivity of the bulk mat, can be measured by a number of standard methods. Typical of these is the "guarded hot plate" method described in A.S.T.M. Method C-177. All of these standard tests, however, are subject to critical limitations. The first limitation is that most of the thermal conductivity measurement methods are essentially "batch" methods as compared to "continuous" methods. That is, they require that a sample of the mat be removed and tested separately away from the main body of the mat. Beside the obvious inconvenience of such a procedure, these methods destroy the integrity of the mat and can produce misleading results if the sample chosen happens not to be representative of the bulk mat. (This situation occurs quite frequently, for the bulk mat is far from homogeneous. There is some variation of physical properties, including density and thermal conductivity, from region to region throughout the mat. The chances are quite high, therefore, that the small sample removed for testing will not be truly representative of the average properties of the mat.)
These testing methods are also quite time consuming. The sample must be removed from the mat and placed in the testing apparatus. The apparatus must be calibrated, if it is a heat flow type apparatus, and brought to stable operating temperatures. Thereafter, the test must be run for a sufficient length of time to obtain good results. The sample is then removed and the apparatus made ready to receive the next sample. The long period of time required to conduct such a test on a single sample obviously severely limits the number of samples which can be tested and therefore also limits the degree to which the overall properties of the entire glass mat can be accurately determined.
It is therefore an object of this invention to provide a method of determining thermal conductivity of fiber mats which could operated on a continuous basis to analyze the entire bulk mat and which could give values averaged over various areas of the mat. Further, such a process should not physically harm or alter the mat. The process of the present invention is a measurement method combining all these desirable properties.